Various abbreviations are used in the present specification. These are listed and explained towards the end of the description.
Mobile networks are currently evolving from pure circuit switched networks towards IP based networks, and by that integrate into IP based infrastructure that is also used for the internet, the World Wide Web and the datacom industry.
Mobile operators that install IMS networks and offer IMS services want to interwork calls between the CS domain and the IMS domain. This is specified in 3GPP for speech, and the responsible node for controlling this interworking is called MGCF. Additionally, interworking with other IP multimedia protocols such as RTSP and H.323 may be implemented in such MGCF.
On top of speech, the CS domain also supports CS video calls by applying the 3G-324M specification. Within the CS core network, a CS video call is a pure CS UDI data call. The control of the video is done using H.245 inband protocol directly between the terminals.
The audio, video, and control channel are multiplexed in order to fit into the single 64 kbps UDI CS data bearer.
FIG. 1 illustrates various nodes involved in a CS video call between two CS terminals 1, using H.245 protocol.
Referring to FIG. 1, each CS terminal 1 is connected to a corresponding M-MGw 2 via a RAN 3. Each M-MGw 2 is controlled by a MSC-S 4. Since the video control protocol H.245 (according to ITU) is used inband, a prerequisite for starting H.245 control signaling is that the UDI bearer is fully established. So the actual video setup starts after the establishment of the CS bearer. Furthermore, the H.245 protocol is rather complex and requires a number of back-and-forth signaling. The result of this is that the call setup time of CS video calls is rather long.
This has been recognized as a problem and recently the concept of MONA (Media Oriented Negotiation Acceleration) has been added to the specifications of CS video calls (ITU-T Recommendation H.324, new Annex K). The main purpose of MONA is to accelerate the setup of CS video calls.
MONA is composed of three protocols and three classes. The protocols are SPC, MPC and ACP. It has been specified that a Class I type terminal shall support SPC, MPC and ACP, that a Class II type terminal shall support MPC and ACP and that a Class III type terminal shall support SPC and ACP.
The initial exchange of preferences between MONA supporting terminals is performed using a PM (Preference Message), which contains signalling information in order to perform MONA procedures. Each terminal will send Preference messages when a call using MONA is initiated.
MPC (Media Preconfigured Channel) is used to quickly setup the multimedia session using any of the preconfigured channels. MPC support, and the setup of the preconfigured channels, is done as part of the PM procedure (using the MPC-TX and MPC-RX bits).
ACP (Accelerated Call Procedure) is available as fallback, in case one of the parties does not support MPC, or it can be used in addition to MPC/SPC in order to add/remove/modify channels, or in case not all channels can be established successfully with MPC. ACP can be viewed as an “optimized H.245 signalling protocol”. It uses H.245 message formats for signalling.
In SPC (Signaling Preconfigured Channel) the predefined SPC channel is used to negotiate the preconfigured audio and video channels to be used for the call. The SPC procedure is used if supported by both terminals and if at least one of the MONA terminals prefers to use SPC, or if the indicated preconfigured channels of both ends do not match.
SPC channel negotiation is done using MOS (Media Oriented Setup) request messages.
In summary, the following problems are associated with pure H.245 processing:                3G-324M requires inband H.245 end-to-end signaling after bearer establishment for negotiation of capability and configuration of codecs and multiplex.        3G-324M requires negotiation of H.223 multiplex level before any H.245 negotiation can take place        Typical 3G-324M roundtrip is at least 250 ms+terminal processing time        Typically 10-12H.245 messages are needed before media can be exchanged, some can be grouped, but requiring at least 3-4 roundtrips        
MONA addresses the above problems.
FIG. 2 (which is similar to FIG. 1) illustrates various nodes involved in a CS video call between two CS terminals, using MONA, and FIG. 3 shows how MONA is integrated into the 3GPP video framework of TS 26.111.
There is currently in 3GPP a technical feasibility study ongoing on how to provide CS-IMS interworking for video calls. The current scope of this feasibility study sketches how to do the video interworking if the H.245 procedures are used for video control. The principles of this are shown in FIG. 4. As shown in FIG. 4, a CS terminal 1 is connected via a RAN to a M-MGw 2, which is controlled by a MSC-S 4. The MSC-S 4 and the M-MGw 2 are connected to a SIP client 6 across a NNI 7, whereby the connection from the MSC-S 4 to the SIP client 6 is via an IMS 5.